The present invention relates generally to the field of substrate processing equipment and methods. More particularly, the present invention relates to a method and apparatus for providing measurement, calibration and control of delivered liquids used to process semiconductors. Merely by way of example, the method and apparatus of the present invention are used to measure, calibrate and control the volume of photoresist, and in some instances the velocity of photoresist, dispensed in a photolithography coating system. The method and apparatus can be applied to other processes for semiconductor substrates, for example those used in the formation of integrated circuits and chemical mechanical polishing.
Modern integrated circuits contain millions of individual elements that are formed by patterning the materials, such as silicon, metal and/or dielectric layers, that make up the integrated circuit to sizes that are small fractions of a micrometer. The technique used throughout the industry for forming such patterns is photolithography. A typical photolithography process sequence generally includes depositing one or more uniform photoresist (resist) layers on the surface of a substrate, drying and curing the deposited layers, patterning the substrate by exposing the photoresist layer to electromagnetic radiation that is suitable for modifying the exposed layer and then developing the patterned photoresist layer.
It is common in the semiconductor industry for many of the steps associated with the photolithography process to be performed in a multi-chamber processing system (e.g., a cluster tool) that has the capability to sequentially process semiconductor wafers in a controlled manner. One example of a cluster tool that is used to deposit (i.e., coat) and develop a photoresist material is commonly referred to as a track lithography tool.
Track lithography tools typically include a mainframe that houses multiple chambers (which are sometimes referred to herein as stations) dedicated to performing the various tasks associated with pre- and post-lithography processing. There are typically both wet and dry processing chambers within track lithography tools. Wet chambers include coat and/or develop bowls, while dry chambers include thermal control units that house bake and/or chill plates. Track lithography tools also frequently include one or more pod/cassette mounting devices, such as an industry standard FOUP (front opening unified pod), to receive substrates from and return substrates to the clean room, multiple substrate transfer robots to transfer substrates between the various chambers/stations of the track tool and an interface that allows the tool to be operatively coupled to a lithography exposure tool in order to transfer substrates into the exposure tool and receive substrates from the exposure tool after the substrates are processed within the exposure tool.
Over the years there has been a strong push within the semiconductor industry to shrink the size of semiconductor devices. The reduced feature sizes have caused the industry's tolerance to process variability to shrink, which in turn, has resulted in semiconductor manufacturing specifications having more stringent requirements for process uniformity and repeatability. An important factor in minimizing process variability during track lithography processing sequences is to ensure that every substrate processed within the track lithography tool for a particular application has the same “wafer history.” A substrate's wafer history is generally monitored and controlled by process engineers to ensure that all of the device fabrication processing variables that may later affect a device's performance are controlled, so that all substrates in the same batch are always processed the same way.
A component of the “wafer history” is the thickness, uniformity, repeatability, and other characteristics of the photolithography chemistry, which includes, without limitation, photoresist, developer, and solvents. Generally, during photolithography processes, a substrate, for example a semiconductor wafer, is rotated on a spin chuck at predetermined speeds while liquids and gases such as solvents, photoresist (resist), developer, and the like are dispensed onto the surface of the substrate. Typically, the wafer history will depend on the process parameters associated with the photolithography process.
As an example, an inadequate volume of photoresist dispensed during a coating operation will generally impact the uniformity and thickness of coatings formed on the substrate. Additionally, the dispense rate of the photoresist will generally impact film properties, including the lateral spreading of the resist in the plane of the substrate. In some instances, therefore, it is desirable to control both the volume and dispense rate of the photoresist applied to the substrate with respect to both the accuracy (e.g., total volume per dispense event) and repeatability (e.g., difference in volume per dispense over a series of dispense events) of the dispense process.
Work in relation to the present invention suggests that the “wafer history” can also include the velocity and diameter of an applied photoresist stream. For example, a larger diameter stream moving at a slower velocity toward the substrate can deliver fluid at the same rate as a small diameter stream moving at a higher velocity. The larger diameter stream presents an increased surface area for a longer period of time, thereby resulting in a greater evaporation of fluid during the dispense which can affect the process. Also, droplets falling or separating in the stream of liquid coming from the nozzle can land on a semiconductor wafer. In some instances such drops may result in defects as the drop “skates” over the surface.
For some photolithography chemical dispense applications, manual calibration of the dispense volume of photoresist applied to the substrate has been provided to improve the accuracy and repeatability of the dispense process. For example, manual calibration of dispense volume can be performed by dispensing a controlled volume into a small pre-weighed cup and moving the cup to a gravimetric scale, or by dispensing the controlled volume into a graduated cylinder. However, these manual calibrations do not provide the level of versatility, automation and speed desirable for current and future track lithography tools. Accordingly, further improvements are desired and are continuously sought by process engineers. Therefore, there is a need in the art for improved methods and apparatus for controlling the dispensed liquids in a photolithography system.